1. Field of the Invention
The present invention relates to an electromagnetic contactor of the type having normally open and normally closed contacts.
2. Description of the Prior Art
The electrical circuits for the bidirectional motors of stamping presses, for example, are typically opened and closed by an electromagnetic contactor or relay.
FIG. 1 of the accompanying drawings shows an exploded perspective view of such a conventional electromagnetic contactor, which includes an attachment base 110 supporting a fixed iron core 114 on which a control coil 112 is mounted. A body frame 116 is screwed to the base and accommodates therein a crossbar 118 which is slidable with respect to the body frame. A movable iron core 120 is attached to the crossbar by a resilient strip 122 in confronting relation to the fixed core 114. Springs 124 are interposed between the crossbar 118 and the base 110 for normally urging the crossbar upwardly to cause the movable core 120 to be spaced upwardly from the fixed core 114.
When an exciting voltage is applied to or removed from the control coil 112, the movable core 120 is brought into or out of contact with the fixed core 114 to thereby slide the crossbar 118 upwardly or downwardly to open or close the electrical circuits of a motor, for example.
Fixed contact members 128 (only one shown) having contacts 126 are secured to the body frame 116. A movable contact member 132 is slidably disposed in an aperture 118a in the crossbar 118 and is biased downwardly by a holding spring 130. The movable contact member has a pair of contacts 134 on opposite ends thereof in confronting relation to the fixed contacts 126.
In response to sliding movements of the crossbar 118, the movable contacts 134 are moved into and out of contact with the fixed contacts 126 to open and close the electrical circuits.
When the fixed and movable contacts are brought into and out of contact with each other, an arc is produced between them. To prevent damage due to arc heating and the arc from flashing outwardly, an arc cover 136 is detachably mounted on the body frame 116.
FIGS. 2 through 4 schematically illustrate the contact arrangements in the prior art electromagnetic contactor. A crossbar 12 is slidably mounted in a base 10 of the electromagnetic contactor. As shown in FIG. 3, the crossbar has a plurality of apertures 14 in each of which a movable contact member 16 is slidably disposed with respect to guide surfaces 18. These guide surfaces may comprise projections or ledges outstanding from the aperture walls or depressions therein, for example, which mate with corresponding notches in or tabs on the movable contact member. Such notches or tabs are preferably extended rearwardly of the contact member to stabilize its sliding movement along the guide surfaces. A holding spring 20 is positioned in each aperture 14 with one end engaging a projecting pedestal 19 and the other end engaging a central portion of the movable contact member 16. The latter has a pair of movable contacts 22 on opposite ends thereof. Fixed contact members 24 are secured to the base 10 and have fixed contacts 26 on their distal ends confronting the respective movable contacts 22. The movable contact member 16 has slides which are slidable along the guide surfaces 18.
The movable and fixed contacts 22, 26 jointly constitute respective contact pairs in the crossbar apertures 14. The contact pairs include as many normally open contacts (FIG. 2) and as many normally closed contacts (FIG. 4) as required for the electrical circuits being controlled.
The crossbar 12 is longitudinally slidable under attractive forces from an electromagnetic means (such as the coil and cores of FIG. 1) to bring the movable and fixed contacts 22, 26 into and out of contact with each other to open and close the electrical circuits.
The distance between the movable and fixed contacts 22, 26 which they must traverse when opening and closing is shorter than the sliding movement stroke of the crossbar 12. With the normally open contact pair as shown in FIG. 2, for example, when the crossbar 12 is driven in the direction of arrow A the movable and fixed contacts 22, 26 are brought into mutual engagement. During the final portion of the sliding movement of the crossbar 12 the movable contact member 16 remains stationary due to its abutting engagement with the fixed contacts 26, and thus merely compresses the spring 20. Stated another way, as the crossbar 12 slides in the direction of arrow A, the movable contact member 16 slides in the direction of arrow B relative to the crossbar, opposite to the direction in which the crossbar is attracted, within the aperture 14, causing the contacts 22, 26 to bear against each other by the compressive force of spring 20.
Conversely, with the normally closed contact pair as shown in FIG. 4, the initial sliding movement of the crossbar 12 in the direction of arrow A merely serves to decompress the spring 20, with the contacts remaining closed, until the movable contact member 16 abuts the wall 30 at the end of the aperture 14. Thereafter the movable contact member is carried in the direction in which the crossbar 12 is attracted to thereby separate the movable and fixed contacts 22, 26 from each other.
The normally open and normally closed contact pairs are prevented from being simultaneously engaged during the sliding movement of the crossbar 12; the normally open contact pairs are closed only after the normally closed contact pairs have been opened. It is essential to prevent such simultaneous closing of the normally open and normally closed contact pairs to avoid the dangerous operation of a stamping press machine, for example. This is achieved by constructing the contactor such that the separation distance between the open contacts 22, 26 in FIG. 2 is greater than the distance between the movable contact member 16 and the aperture end wall 30 in FIG. 4.
As described above, the electrical circuits are opened and closed in response to contacting and separating movements of the normally open and normally closed contact pairs, and the guide surfaces 18 extend parallel to the longitudinal axis of the crossbar 12. If one of the contact pairs becomes fused, however, the movable contact member 16 tends to move by the decompression travel of the spring 20 and flex in the direction of sliding movement of the crossbar as the latter slides at the time the fused contacts are to be separated. With electromagnetic contactors of the type in which the distance between the open fixed and movable contact pairs is relatively short, the extent to which the movable contact member 16 moves by the decompression travel of the spring 20 and flexes may exceed the contact separation distance less the decompression travel of the spring 20. Such a condition results in the normally open and normally closed contact pairs being simultaneously closed, thus making the press device or the like being controlled more susceptible to abnormal or dangerous conditions.